1. Field of the Invention
The present invention relates to a driving force controlling apparatus of a vehicle.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 6-289039 discusses a slip amount calculating device of a vehicle. In the slip amount calculating device, when rotational speed sensors for four wheels all operate normally, a slip amount is calculated on the basis of an average value of rotational speeds of left and right idle wheels and an average value of rotational speeds of left and right driving wheels. In addition, when an abnormality occurs in the rotational speed sensor of a right front wheel or in the rotational speed sensor of a right rear wheel, a slip amount is calculated on the basis of the rotational speed of a left front wheel and the rotational speed of a left rear wheel. Further, when an abnormality occurs in the rotational speed sensor of the left front wheel or the rotational speed sensor of the left rear wheel, a slip amount is calculated on the basis of the rotational speed of the right front wheel and a rotational speed of the right rear wheel.
However, in a four-wheel driving vehicle in which the front wheels are main driving wheels and the rear wheels are sub-driving wheels, although any abnormality in front-wheel speed sensors can be precisely determined by comparing outputs of the sensors with a rotational speed of an output shaft of a transmission, it is difficult to easily determine precisely any abnormality in rear-wheel speed sensors for which such a method cannot be used.
Therefore, when it is determined that a road surface is a low-friction-coefficient road surface using abnormal rear-wheel speeds output from the rear-wheel speed sensors, an erroneous determination may occur. When driving force control, such as limiting of a shift change of the transmission or fuel cut of an engine is performed on the basis of a erroneous determination result, an excessive torque is input to the transmission, thereby causing damage to a vehicle or hindering drivability of the vehicle.